A. Field of the Invention
The present invention concerns a packaging material with good gas barrier properties, also in addition a method of producing the material.
B. Description of Relevant Art
Foodstuffs and other types of oxidization and storage sensitive products already spoil or deteriorate after a very short time if they are exposed to the action of oxygen. In order to be able to package and keep the product for at least a certain time with maintained fresh qualities it is thus required that the product should be packaged in a packaging which is difficult to penetrate or best of all completely impenetrable for oxygen which would otherwise easily be able to penetrate into the packaging and cause the product to deteriorate.
A well known example of a packaging material which has good gas barrier properties and which is often used in packagings for liquid foodstuffs of the type such as juice, wine, edible oil etc. is built up of layers of paper or cardboard and plastic with a gas barrier layer of aluminum (Al foil) applied to one side of the paper or cardboard layer. An Al foil is in itself completely impenetrable for oxygen and has also other valuable barrier properties for the product packaged. e.g. to penetrability by light, at the same time as during the forming of the material into packagings it makes it possible to execute tight and mechanically strong joints in the material through inductive heating. The problem with Al foil has, however, always been that it is expensive and that it is practically inelastic which has often entailed that it has cracked or burst in regions of material which have been exposed to particularly strong tensile strains during production of the packaging. There has therefore been a wish to find a cheaper but just as effective alternative to Al foil.
The Swedish patent no. 8200761-8 gives details of a packaging material completely free from Al foil with good barrier properties. This known material comprises a skeletal layer of paper or cardboard and a gas barrier layer of polyvinyl alcohol (PVAL) applied to one side of the-skeletal layer. A layer of PVAL has, as is known, good gas-tight properties so long as the polymer layer is dry, but quickly loses its barrier properties in proportion as the moisture content in the layer increases. In order to protect the PVAL layer against moisture it is therefore proposed in the Swedish patent that the outside of the PVAL layer should be covered with plastic, preferably polyethylene. According to the Swedish patent the packaging material is produced through an aqueous dispersion or aqueous emulsion of PVAL being applied along one side of a strip or a sheet of paper or cardboard, after which "the surplus water" in the layer applied is driven away through heating to form a dry, coherent PVAL layer. The PVAL layer and the other side of the strip or sheet are subsequently coated with the moisture-repellent plastic coatings which are extruded on to the two sides of the strip or sheet.
The disadvantage with the procedure described in the Swedish patent is that PVAL cannot be applied as a pure aqueous dispersion or aqueous emulsion but requires at least one further chemical component or thickener in order to avoid the water in the dispersion or emulsion applied being sucked into and absorbed by the water-absorbent material in the strip or sheet. Another disadvantage which is at least partly connected with the water-absorbent properties of the sheet or strip material is that the PVAL layer applied must be dried through heating in order to adjust the correct moisture content in the PVAL layer, which makes the known procedure energy and therefore cost intensive.